1. Field of the Invention
The present invention relates to an apparatus mounted on a wrist of an industrial robot and intended to perform a locus control for a small-locus machining tool.
2. Description of the Prior Art
In recent years, there is an increasing demand for performing small hole machining, such as perforating option holes in an object such as an automobile body, wherein the object has gone through a bending process. It is further desired that such small hole machining is particularly performed within a line using robots.
In a small-locus machining tool for small hole machining, such as a weaving locus machining tool for arc welding or a marking operation, the locus accuracy of the tool when using a robot will be a problem, though there may arise a variety of other problems in performing such type of machining using the industrial robot. For example, in general, it is difficult to move the tool along a desired path or locus by controlling the distal end of the arm because of a larger inertia and a lower rigidity of the distal end of the robot arm on which a tool is mounted. Also, in the case of a multi-spindle robot such as a six-spindle robot, the amount of movement is determined by calculating the interpolation of the move command. Therefore, if the interpolating period in too long, the interval of the interpolation relative to the length of the locus becomes too rough, especially in the case of a small path, thereby making it difficult for the tool to accurately follow the locus as required.
Hence, the above problem has conventionally been solved by attaching a small-locus machining apparatus having a driving control to the distal end of the wrist of the robot, thereby enabling the tool to move more finely via the wrist of the robot for higher locus accuracy. FIGS. 7 to 10 illustrate examples of such a small-locus machining apparatus.
FIG. 7 is a diagram showing a a small-locus machining apparatus 3 for controlling the weaving locus in arc welding, the apparatus 3 being mounted on the distal end of the arm of a robot 1. In this small-locus machining apparatus 3, the spindles for displacing the tool along a locus are controlled by calculating the interpolation, independently of the robot body 1. The small-locus machining apparatus 3 shown in prior art FIG. 8 is controlled so that the move command is interpolated with respect to two orthogonal spindles to move the tool (marker 2) for the marking operation, independently of the operation of the robot body 1.
However, the small-locus machining apparatus using such orthogonal spindles involves the following disadvantages:
First, the combination of the robot body and the small-locus machining apparatus causes an increase in size to enlarge the interference region with the periphery, thereby increasing the load applied onto the robot body 1. For the linear spindles constituting the orthogonal spindles, it is also difficult for their lubricating sections to be sealed effectively. Regarding the first and second motors for controllably driving the first and second spindles, the second motor is mounted on the moving part which is driven by the first spindle, so that the first spindle is subjected to an increased load, which in turn makes it difficult to arrange the cables.
FIGS. 9 and 10 depict a prior art compass mechanism for describing a circular locus by using a spindle T1 for circular movement of the tool and also using a spindle T2 for radial movement in the circle formed around the spindle T1, independently of the operation of the robot body. Naturally, it is possible to cause the tool to describe any locus other than a circle by so commanding the two spindles T1 and T2. Reference marks M1 and M2 denote servomotors for driving spindle T1 and spindle T2, respectively.
However, such a compass mechanism generally entails the following disadvantages:
Gearing, which is shown as a gear 4 in FIG. 9, is used in the final speed reduction stage for transmitting the power to the spindles T1 and T2, so that, when generating a configuration other than the circular locus, it may be influenced by backlash, which may make it impossible to secure a location with predetermined accuracy.
An attempt to eliminate this backlash may cause an increase in both complication and dimension of the apparatus and may result in an increase in cost.
One step deceleration using gears inevitably requires a motor to have a higher output since a desired speed reduction ratio is not ensured. This entails an increase in the dimensions of the apparatus, as well as cost and power consumption. Also, a larger gear pitch may bring about a deterioration in the interpolation accuracy.
On the other hand, an attempt to secure a desired speed reduction ratio by employing the multi-gears may be followed by increased weight, an enlarged apparatus, accumulated backlash, and increased cost.
In the case where this compass mechanism is applied to a laser machining tool such as a small hole machining tool, the connection between the tool 2 and the optical fiber cable 5 will be confined within a rotation mechanism, which is difficult to disassemble, in the body, thus making daily maintenance work difficult for the optical system.
Similarly, in the case of an application to laser machining, the optical system passes through the inside of a movable part, which performs a double-rotation of the compass mechanism. It is however difficult to perfectly seal the lubricating part of the movable section, and thus difficult to protect the optical system from being contaminated by the lubricating part.
In the case of a compass mechanism, in order to prevent the cable 5 leading to the tool 2 from being excessively twisted by one turn of the tool 2 performing a circular locus movement, the tool 2 must be supported by, for example, a bearing 6, and a whirl-stop 7 must be provided.
It is to be appreciated that the small-locus machining apparatus including the above compass mechanism is usually provided with a control means comprising means for detecting the amount of movement, for calculating the interpolation, and for carrying out the results.